Semiconductor laser and fabricating method therefor

ABSTRACT

At least a lower cladding layer, an active layer for generating laser light, a first upper cladding layer, an etching stopper layer and a second upper cladding layer are stacked on a substrate. An impurity for restraining laser light absorption is diffused into the second upper cladding layer along a region where a light-emitting end surface is to be formed, under a condition that allows the etching stopper layer to maintain a function of stopping etching for the second upper cladding layer (First annealing process). Etching is performed until the etching stopper layer is reached such that the second upper cladding layer is left in a ridge shape. The impurity in the second upper cladding layer is re-diffused into the active layer to thereby cause local intermixing of the active layer in a portion extending along the light-emitting end surface and located just under the ridge (Second annealing process).

BACKGROUND OF THE INVENTION

[0001] The present invention generally relates to semiconductor lasersand more particularly to a semiconductor laser that has a window regionof a small quantity of laser light absorption at its light-emitting endsurfaces. A semiconductor laser of this type is applied to an opticaldisk drive and the like that requires a high output.

[0002] The present invention also relates to a semiconductor laserfabricating method capable of fabricating the semiconductor laser of theabove-mentioned type with high accuracy.

[0003] In the high-output semiconductor laser for use in an optical diskdrive or the like, the light-emitting end surface sometimes deterioratesdue to high density of light, possibly causing damage called COD(Catastrophic Optical Damage). As a measure against this, it has beenproposed to provide the light-emitting end surfaces with a window regionthat absorbs less laser light than the inside of the active layer does.

[0004] As a conventional high-output semiconductor laser that has awindow region at its light-emitting end surfaces, there is one as shownin FIG. 20 (see WO96/11503). This semiconductor laser has on an n-typeGaAs substrate 1 an n-conductivity type buffer layer 11, ann-conductivity type first cladding layer 2′, a first separateconfinement layer 2″, an active layer 3, a second separate confinementlayer 4″, a p-conductivity type second cladding layer 4′ and an etchingstopper layer (having a thickness of 0.01 μm) 5. A p-conductivity typesecond cladding layer 4 ⁰, a p-conductivity type intermediate layer 9and a p-conductivity type first contact layer 10 are provided on thisetching stopper layer 5 so as to constitute a mesa 12 that extends in astriped shape in the direction of line XXI-XXI in FIG. 20. Regions atboth sides of the mesa 12 are filled with an n-type current blockinglayer 13. A second contact layer 6 and an electrode (connectionconductor) 7 are provided over the mesa 12 and the n-type currentblocking layer 13. On the other hand, an electrode (connectionconductor) 8 is formed over the rear surface of the n-type GaAssubstrate 1.

[0005] As shown in FIG. 21 (showing a cross section taken along the lineXXI-XXI of FIG. 20), the active layer 3 is constructed of a laminate oftwo quantum well layers 3′ and a barrier layer 3″ therebetween.Portions, which belong to the active layer 3 and are located nearlight-emitting end surfaces (exit surfaces) 50 and 51, serve as windowregions (passive regions) 3B where the laser light absorption is lessthan in the active layer inside 3A.

[0006] This semiconductor laser is fabricated as follows. As shown inFIG. 22, the layers of the n-conductivity type buffer layer 11 throughthe contact layer 10 are first grown on the n-type GaAs substrate 1 byOMVPE (organometallic vapor phase epitaxy). Next, a masking layer 30made of silicon oxide is formed so as to have opening portions 31 and 32along the light-emitting end surfaces 50 and 51. The wafer in this stateis introduced in a closed capsule together with zinc arsenide and thecapsule is heated to a temperature of 600° C., so that Zn atoms 59diffuse from the upper surface side of the contact layer 10 beyond theactive layer 3. Through these processes, local intermixing of the activelayer 3 (namely making a part of the active layer 3 a mixed crystal)takes place at the portions near the light-emitting end surfaces 50 and51, which serve as the window regions 3B where the energy bandgap isgreater and accordingly the laser light absorption is less than in theactive layer inside 3A. After the mask 30 is removed, a strip-shapedmask 40 is formed, which extends perpendicularly to the light-emittingend surfaces 50 and 51, as shown in FIG. 23. Next, the mesa 12 is formedjust under the mask 40 by etching the semiconductor layers 10, 9, and 4⁰ at portions on both sides of the mask 40 until the etching stopperlayer 5 is reached. Subsequently, as shown in FIG. 20, the blockinglayer 13 is formed on both sides of the mesa 12 by OMVPE. Afterplanarizing the blocking layer and removing the mask 40, the secondcontact layer 6 is formed by using the OMVPE method again. Then, theelectrodes 7 and 8 are formed over the upper surface of the contactlayer 6 and the lower surface of the substrate 1, respectively (thefabrication completed).

[0007] According to the aforementioned fabricating method, during thestep of forming the window regions (passive regions) 3B throughintermixing of the active layer 3 by diffusion of impurity, intermixingof the etching stopper layer 5 may also take place. Then, there will bea problem that the etching stopper layer 5 and the second cladding layer(lower portion) 4′ are etched in the process of forming the mesa 12,which leads to reduction of the processing accuracy of the mesa 12. Ifthe etching progresses extremely, there may arise a further problem thatthe current blocking layer 13 and the n-type cladding layer 11 aredisadvantageously electrically short-circuited. On the other hand, ifthe annealing temperature and time are reduced to avoid these problemsrelated to the fabricating process, then there may conversely arise aproblem that sufficient intermixing does not take place in the windowregion 3B, resulting in difficulties in obtaining the effect ofrestraining photoabsorption.

SUMMARY OF THE INVENTION

[0008] Accordingly, it is an object of this invention to provide asemiconductor laser which has a window region in its light-emitting endsurfaces and is able to be easily fabricated with high accuracy.

[0009] Another object of this invention is to provide a method foreasily fabricating a semiconductor laser having a window region in itslight-emitting end surfaces, with high accuracy.

[0010] In order to accomplish the first object, there is provided,according to an aspect of the present invention, a semiconductor laser,which emits laser light through a light-emitting end surface,comprising:

[0011] a lower cladding layer, an active layer for generating laserlight, a first upper cladding layer and an etching stopper layer stackedin this order on a substrate;

[0012] a second upper cladding layer formed in a shape of a ridge on theetching stopper layer, the ridge extending perpendicularly to thelight-emitting end surface;

[0013] a current blocking layer disposed in regions on both sides of thesecond upper cladding layer; and

[0014] an impurity diffused in a portion extending along thelight-emitting end surface from the etching stopper layer to the activelayer and located at least under the ridge for local intermixing in thisportion to restrain laser light absorption, wherein

[0015] in a region along the light-emitting end surface, the etchingstopper layer has a bandgap smaller in portions thereof disposed inpositions corresponding to both sides of the ridge than in a portionthereof located just under the ridge.

[0016] In the semiconductor laser of the present invention, in theregion along the light-emitting end surface, the energy bandgap of theportions, of the etching stopper layer, that correspond to both sides ofthe ridge is smaller than the energy bandgap of the portion, of theetching stopper layer, that is located just under the ridge. Therefore,in the region along the light-emitting end surface, the portionscorresponding to both sides of the ridge of the etching stopper layercan effectively fulfill the function to stop the etching when the secondupper cladding layer is formed in a ridge shape on the etching stopperlayer. Therefore, this semiconductor laser is easily fabricated withhigh accuracy.

[0017] In one embodiment, in the region along the light-emitting endsurface, the active layer has a bandgap larger in a portion thereoflocated just under the ridge than in portions thereof disposed inpositions corresponding to both sides of the ridge.

[0018] Accordingly, the portion, which belongs to the active layer andis located just under the ridge in the region along the light-emittingend surface, can effectively restrain the COD, serving as a windowregion. Moreover, because intermixing does not take place in an internalregion of the active layer, the fabrication becomes easy. It is to benoted that the problem of COD does not occur in the internal area of theactive layer, so that the internal area of the active layer is notrequired to be intermixed.

[0019] In one embodiment, in the region along the light-emitting endsurface, a photoluminescence wavelength shift to a shorter wavelengthside due to the local intermixing of the active layer in the portionlocated just under the ridge is 18 nm or more. Therefore, the maximumoptical output is increased by 1.41 times or more in comparison with theconventional semiconductor laser. Moreover, a photoluminescencewavelength shift to the shorter wavelength side due to the localintermixing of the active layer in the portions corresponding to bothsides of the ridge is not larger than 15 nm. Therefore, fabricationbecomes extremely easy.

[0020] In one embodiment, the first upper cladding layer contains adiffused impurity of Be or C, and the impurity diffused in said portionextending along the light-emitting end surface from the etching stopperlayer to the active layer is Zn.

[0021] The Zn atoms easily diffuse, and the diffusion easily causes theintermixing of the active layer. Moreover, the elements Be and C havediffusion constants smaller than that of Zn. Therefore, the Zn atoms caneasily be diffused into the active layer while avoiding the phenomenonof the diffusion of the diffused impurity (Be or C) contained in thefirst upper cladding layer into the active layer. Therefore, thesemiconductor laser is easy to fabricate.

[0022] In one embodiment, the second upper cladding layer contains adiffused impurity of Be or C.

[0023] The elements of Be and C have diffusion constants smaller thanthat of Zn. Therefore, Zn atoms can easily be diffused into the activelayer while avoiding the phenomenon of the diffusion of the diffusedimpurity (Be or C) contained in the second upper cladding layer into theactive layer. Therefore, the semiconductor laser is easy to fabricate.

[0024] In one embodiment, the active layer comprises at least onequantum well layer and barrier layers alternating with the quantum welllayer. The at least one quantum well layer is constructed of(Al_(x)Ga_(1−x))_(y)In_(1−y)P (0≦x≦1 and 0≦y≦1), and the barrier layersare constructed of (Al_(x)Ga_(1−x))_(y)In_(1−y)P (0≦x≦1 and 0≦y≦1) whoseAl content (x) is greater than that of the quantum well layer.

[0025] The substance of (Al_(x)Ga_(1−x))_(y)In_(1−y)P that constitutesthe quantum well layer and the barrier layers is easily intermixed evenwhen the concentration of Zn atoms to be diffused is on the order of acomparatively low value of 10¹⁸ cm⁻³. Therefore, the semiconductor laseris easy to fabricate.

[0026] It should be understood that although the letters of x, y and zare herein used for expressing the compositions of the compoundsemiconductors, x, y and z can take different values in each of thecompound semiconductors.

[0027] In one embodiment, the etching stopper layer is constructed ofGa_(y)In_(1−y)P (0≦y≦1), and the first and second upper cladding layersare each constructed of (Al_(x)Ga_(1−x))_(y)In_(1−y)P (0≦x≦1 and 0≦y≦1).

[0028] The substance of Ga_(y)In_(1−y)P (0≦y≦1), which constitutes theetching stopper layer, allows the substance of(Al_(x)Ga_(1−x))_(y)In_(1−y)P (0≦x≦1 and 0≦y≦1), which constitutes thefirst and second upper cladding layers, to be selectively left when thelatter substance is removed by wet etching. Therefore, the second uppercladding layer is easily formed in a ridge shape on the etching stopperlayer.

[0029] In one embodiment, the active layer comprises at least onequantum well layer and barrier layers alternating with the quantum welllayer. The at least one quantum well layer is constructed ofIn_(z)Ga_(1−z)As (0≦z≦1) or Al_(x)Ga_(1−x)As (0≦x≦1), and the barrierlayers are constructed of Al_(x)Ga_(1−x)As (0≦x≦1) whose Al content (x)is greater than that of the quantum well layer when the latter isconstructed of Al_(x)Ga_(1−x)As (0≦x≦1).

[0030] The substance of In_(z)Ga_(1−z)As (0≦z≦1) and Al_(x)Ga_(1−x)As(0≦x≦1), either of which constitutes the quantum well layer or layers,and the substance of Al_(x)Ga_(1−x)As (0≦x≦1), which constitutes thebarrier layers, are both easily be intermixed by the diffusion of Znatoms. Therefore, the semiconductor laser is easy to fabricate.

[0031] In one embodiment, the etching stopper layer is constructed ofAl_(x)Ga_(1−x)As (0≦x≦0.3), and the first and second upper claddinglayers are each constructed of Al_(y)Ga_(1−y)As (x<y≦1).

[0032] The substance of Al_(x)Ga_(1−x)As (0≦x≦0.3), which constitutesthe etching stopper layer, allows the substance of(Al_(x)Ga_(1−x))_(y)In_(1−y)P (0≦x≦1 and 0≦y≦1), which constitutes thefirst and second upper cladding layers, to be selectively left when thelatter substance is removed by wet etching. Therefore, the second uppercladding layer is easily formed in a ridge shape on the etching stopperlayer.

[0033] In order to accomplish the second object, there is provided,according to another aspect of the present invention, a method forfabricating a semiconductor laser that emits laser light through alight-emitting end surface, comprising:

[0034] a process for forming at least a lower cladding layer, an activelayer for generating laser light, a first upper cladding layer, anetching stopper layer and a second upper cladding layer in this order ona substrate;

[0035] a first annealing process for diffusing an impurity forrestraining laser light absorption into the second upper cladding layeralong a region where a light-emitting end surface is to be formed, undera condition that allows the etching stopper layer to maintain a functionof stopping etching for the second upper cladding layer;

[0036] an etching process for performing etching until the etchingstopper layer is reached such that the second upper cladding layer isleft in a ridge shape extending perpendicularly to the light-emittingend surface to be formed; and

[0037] a second annealing process for re-diffusing the impurity oncediffused in the region of the ridge-shaped second upper cladding layerwhere the light-emitting end surface is to be formed, into the activelayer through the etching stopper layer to thereby cause localintermixing of the active layer in a portion that extends along thelight-emitting end surface to be formed and is located just under theridge.

[0038] According to the semiconductor laser fabricating method of thepresent invention, as shown by example in FIG. 18A, at least a lowercladding layer 71, an active layer 72 for generating laser light, afirst upper cladding layer 73, an etching stopper layer 74 and a secondupper cladding layer 76 are stacked in this order on a substrate. Thefirst annealing process is carried out under the condition that allowsthe etching stopper layer to maintain the function of stopping theetching for the second upper cladding layer, or for example, under thecondition of a low temperature or a short time. As shown by example inFIG. 18B, an impurity 89 is diffused from, for example, a soliddiffusion source 81 to the second upper cladding layer 76, whereas theimpurity is substantially not diffused to the etching stopper layer 74.Therefore, in the etching process for processing the second uppercladding layer 76 in a ridge shape, the etching stopper layer 74 caneffectively stop the etching as shown by example in FIG. 18C. As aresult, the processing accuracy of the ridge is increased, and theelectrical short-circuit between layers is prevented. Moreover, in thesecond annealing process, as shown by example in FIGS. 19A and 19B,sufficient local intermixing of the active layer takes place in aportion 72B that extends along the light-emitting end surface to beformed and is located just under the ridge 76. This portion 72B wherethe intermixing took place operates as a window region absorbing littlelaser light at the light-emitting end surface after the completion ofthe semiconductor laser, allowing the COD to be restrained. Thesemiconductor laser fabricating method with the above-mentionedarrangement can easily fabricate a semiconductor laser having a windowregion at the light-emitting end surface with high accuracy.

[0039] In one embodiment, a photoluminescence wavelength shift to ashorter wavelength side through the first annealing process at a portionof the active layer that extends along the region where thelight-emitting end surface is to be formed is not larger than 15 nm.Therefore, intermixing of the etching stopper layer hardly occurs in thefirst annealing process. Therefore, after the first annealing process,the function of the etching stopper layer to stop the etching for thesecond upper cladding layer can be maintained. Furthermore, aphotoluminescence wavelength shift to the shorter wavelength sidethrough the second annealing process at the portion of the active layerthat extends along the region where the light-emitting end surface is tobe formed and is located just under the ridge is 18 nm or larger. Thismeans that sufficient local intermixing of the active layer took placeat that portion in the second annealing process. This portion in whichintermixing took place operates as a window region of a small quantityof laser light absorption at the light-emitting end surface after thecompletion of the semiconductor laser, allowing the COD to berestrained.

[0040] In one embodiment, after the etching process and before thesecond annealing process, the method further includes providing on thesubstrate an impurity evaporation preventing layer for preventing theimpurity from evaporating from the second upper cladding layer to theoutside.

[0041] In the state in which nothing is provided on the second uppercladding layer as shown by example in FIG. 19A, the impurity 89 will bepartially evaporated from the second upper cladding layer 76 to theoutside during the second annealing process. In contrast to this, asshown by example in FIG. 19B, according to the semiconductor laserfabricating method of this embodiment, the second annealing process iscarried out in a state in which an impurity evaporation preventing layer85 for preventing the evaporation of the impurity 89 from the secondupper cladding layer 76 to the outside is provided. As a result, theevaporation of the impurity 89 from the second upper cladding layer 76to the outside can be prevented. Therefore, the intermixing through thesecond annealing process of the active layer in the portion 72B, whichextends along the region where the light-emitting end surface is to beformed and is located just under the ridge, can be further promoted.

[0042] In one embodiment, the impurity evaporation preventing layer ismade of silicon oxide, silicon nitride, or alumina.

[0043] The substance of silicon oxide, silicon nitride or alumina isfine and dense and therefore suitable for preventing the evaporation ofthe impurity. Moreover, the substance of silicon oxide, silicon nitrideor alumina can selectively be removed by an etchant that does not erodea semiconductor underlayer. Therefore, this method can easily fabricatethe semiconductor laser with high accuracy.

[0044] Generally in the semiconductor laser, a compound semiconductorlayer, such as a current blocking layer or the like, is formed on theetching stopper layer. In one embodiment, such a compound semiconductorlayer is utilized as the impurity evaporation preventing layer.Therefore, the fabricating process can be simplified.

[0045] In one embodiment, the conductor semiconductor layer serving asthe impurity evaporation preventing layer is of a conductivity typedifferent from that of second upper cladding layer. The conductorsemiconductor layer can be utilized to form a current blocking layer forrestraining a wattless current.

[0046] In one embodiment, the impurity evaporation preventing layer isformed at a temperature lower than a temperature at which the impurityis re-diffused in the second annealing process.

[0047] According to the semiconductor laser fabricating method of thisembodiment, the impurity is prevented from evaporating during formationof the impurity evaporation preventing layer, due to the temperature atwhich the impurity evaporation preventing layer is formed.

[0048] In one embodiment, the first upper cladding layer contains adiffused impurity of Be or C, and the impurity diffused in the portionof the active layer that extends along the region where thelight-emitting end surface is to be formed and is located just under theridge is Zn.

[0049] In one embodiment, the diffused impurity contained in the secondupper cladding layer is Be or C.

[0050] In one embodiment, the active layer is formed by alternating atleast one quantum well layer and barrier layers. The at least onequantum well layer is constructed of (Al_(x)Ga_(1−x))_(y)In_(1−y)P(0≦x≦1 and 0≦y≦1), the barrier layers are constructed of(Al_(x)Ga_(1−x))_(y)In_(1−y)P (0≦x≦1 and 0≦y≦1) whose Al content (x) isgreater than that of the quantum well layer. The first annealing processis carried out under a condition of a temperature of 450° C. to 570° C.for 10 minutes or more or a temperature of 550° C. to 650° C. for 10minutes or less. And, the second annealing process is carried out undera condition of a temperature of 570° C. to 750° C. for 10 minutes ormore or a temperature of 650° C. to 850° C. for 10 minutes or less.

[0051] Under the condition of the first annealing process, when theetching stopper layer is constructed of Ga_(y)In_(1−y)P (0≦y≦1),intermixing of the etching stopper layer does not take place. Moreover,under the condition of the second annealing process, the active layer issatisfactorily activated. However, under a temperature condition that ishigher than the temperature condition of the second annealing process,the diffused impurity (p-type, in particular) in the second uppercladding layer disadvantageously diffuses into regions of the activelayer other than the region near the light-emitting end surface,possibly deteriorating the characteristics of a laser oscillationthreshold value and so on.

[0052] The first annealing process under the conditions of a temperatureof 450° C. to 570° C. and ten minutes or more can be carried out byusing an ordinary annealing furnace, while the first annealing processunder the conditions of a temperature of 550° C. to 650° C. and tenminutes or less can be carried out by using the RTA (Rapid ThermalAnnealing) method.

[0053] Molecular beam epitaxy is a semiconductor forming technique to becarried out in a high vacuum without using hydrogen. Therefore, if thesecond annealing process is carried out by raising a substratetemperature when a semiconductor layer is formed by molecular beamepitaxy, then the diffused impurity inactivation due to the mixture ofhydrogen can be restrained.

[0054] In one embodiment, the second annealing process is carried out ina nitrogen ambient. According to this embodiment, the diffused impurityinactivation due to the mixture of hydrogen can be restrained.

BRIEF DESCRIPTION OF THE DRAWINGS

[0055] The present invention will become more fully understood from thedetailed description given hereinbelow and the accompanying drawingswhich are given by way of illustration only, and thus are not limitativeof the present invention, and wherein:

[0056]FIG. 1 is a perspective view of a semiconductor laser according toa first embodiment of the present invention;

[0057]FIG. 2 is a sectional view taken along line II-II of FIG. 1;

[0058]FIG. 3 is a sectional view taken along line III-III of FIG. 1;

[0059] FIGS. 4A-4E are views for explaining the fabricating processsteps for the semiconductor laser of the first embodiment of the presentinvention;

[0060]FIG. 5 is a graph showing the results of an active layerphotoluminescence wavelength shift when a first annealing condition ischanged;

[0061]FIG. 6 is a graph showing the results of the active layerphotoluminescence wavelength shift when the first annealing condition ischanged;

[0062]FIG. 7 is a graph showing the results of the active layerphotoluminescence wavelength shift when the first annealing condition ischanged;

[0063]FIG. 8 is a graph showing a Zn concentration profile in thedirection of depth of a ridge portion in the first embodiment;

[0064]FIG. 9 is an explanatory view showing the relationship between theamount of wavelength shift of the active layer and the maximum opticaloutput of the semiconductor laser of the first embodiment;

[0065]FIG. 10 is a perspective view of a semiconductor laser accordingto a second embodiment of the present invention;

[0066]FIG. 11 is a sectional view taken along line XI-XI of FIG. 10;

[0067]FIG. 12 is a sectional view taken along line XII-XII of FIG. 10;

[0068] FIGS. 13A-13E are views showing the fabricating process steps forthe semiconductor laser of the second embodiment of the presentinvention;

[0069]FIG. 14 is a perspective view of a semiconductor laser accordingto a third embodiment of the present invention;

[0070]FIG. 15 is a sectional view taken along line XV-XV of FIG. 14;

[0071]FIG. 16 is a sectional view taken along line XVI-XVI of FIG. 14;

[0072] FIGS. 17A-17E are views showing the fabricating process steps forthe semiconductor laser of the third embodiment of the presentinvention;

[0073]FIGS. 18A, 18B and 18C are views schematically showing how theimpurity diffuses when the semiconductor laser fabricating method of thepresent invention is used;

[0074]FIGS. 19A and 19B are views schematically showing how the impuritydiffuses when the semiconductor laser fabricating method of the presentinvention is used;

[0075]FIG. 20 is a perspective view of a prior art semiconductor laser;

[0076]FIG. 21 is a sectional view taken along line XXI-XXI of FIG. 23;

[0077]FIG. 22 is a view for explaining the fabricating process of theprior art semiconductor laser; and

[0078]FIG. 23 is a view for explaining the fabricating process of theprior art semiconductor laser.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0079] The present invention will be described in detail below on thebasis of the embodiments shown in the drawings.

[0080] Hereinafter, the substances of (Al_(x)Ga_(1−x))_(y)In_(1−y)P(0≦x≦1 and 0≦y≦1), Ga_(y)In_(1−y)P (0≦y≦1) and Al_(x)Ga_(1−x)As (0≦x≦1)will sometimes be abbreviated to AlGaInP, GaInP and AlGaAs,respectively.

[0081] (First Embodiment)

[0082]FIG. 1 shows the device structure of an end-surface window typesemiconductor laser of a first embodiment of the present invention. FIG.2 shows a cross section taken along line II-II of FIG. 1, and FIG. 3shows a cross section taken along line III-III of FIG. 1.

[0083] As shown in FIG. 1, an n-type AlGaInP lower cladding layer 101,an active layer 102 for generating laser light, a p-type AlGaInP firstupper cladding layer 103 and a p-type GaInP etching stopper layer 104are stacked in this order on an n-type GaAs substrate 100. The activelayer 102 is formed by alternating undoped quantum well layers withbarrier layers. A ridge 105, which extends in a stripe shapeperpendicularly to light-emitting end surfaces 150 and 151, isconstructed of a p-type AlGaInP second upper cladding layer 106 and ap-type GaAs cap layer 107. An n-type AlInP current blocking layer 108 isformed in regions at both sides of the ridge 105. As is apparent fromFIG. 2, this current blocking layer 108 extends also over the ridge 105(the extended portion is indicated by 108B) in regions located near thelight-emitting end surfaces 150 and 151 and covers those portions 102Bof the active layer 102 that are located near the light-emitting endsurfaces 150 and 151. With this arrangement, a wattless current isprevented from being injected into the portions 102B of the active layer102 adjacent to the light-emitting end surfaces 150 and 151. Moreover,as is apparent from FIGS. 2 and 3, the p-type GaAs cap layer 107 and ap-type GaAs contact layer 110 are in contact and electrically connectedwith each other in an internal region other than the regions locatednear the light-emitting end surfaces 150 and 151 with regard to theII-II direction.

[0084] In this semiconductor laser, regarding regions extending alongthe light-emitting end surfaces 150 and 151, the portions 102B of theactive layer 102 that are located just under the ridge 105 have anenergy bandgap greater than the energy bandgap of the other portions ofthe active layer 102 that are located in positions corresponding to bothsides of the ridge 105. Thus, the portions 102B serve as window regionsabsorbing little laser light.

[0085] This semiconductor laser is fabricated as follows.

[0086] First of all, as shown in FIG. 4A, on an n-type GaAs substrate100, there are formed by MBE (molecular beam epitaxy) an n-type(Al_(0.7)Ga_(0.3))_(0.5)In_(0.5)P lower cladding layer 101 (having acarrier density of 1×10¹⁸ cm⁻³), an active layer 102 that has astructure in which three undoped GaInP layers (having a thickness of 6nm) alternate with four undoped (Al_(0.5)Ga_(0.5))_(0.5)In_(0.5)P layers(having a thickness of 8 nm) with one undoped GaInP layer being heldbetween adjacent two undoped (Al_(0.5)Ga_(0.5))_(0.5)In_(0.5)P layers,the p-type (Al_(0.7)Ga_(0.3))_(0.5)In_(0.5)P first upper cladding layer103 (having a carrier density of 1.5×10¹⁸ cm⁻³), a p-typeGa_(0.6)In_(0.4)P etching stopper layer 104 (having a thickness of 6 nmand a carrier density of 1.5×10¹⁸ cm⁻³) a p-type(Al_(0.7)Ga_(0.3))_(0.5)In_(0.5)P second upper cladding layer 106(having a thickness of 0.2 μm and a carrier density of 2×10¹⁸ cm⁻³) anda p-type GaAs cap layer 107 (having a carrier density of 3×10¹⁸ cm⁻³) inthis order. In this case, the n-type dopant is Si, and the p-type dopantis Be.

[0087] Next, as shown in FIG. 4B, a ZnO (zinc oxide) layer 131 having athickness of 50 nm, which serves as an impurity diffusion source, isformed in a stripe shape on the cap layer 107 along the regions wherethe light-emitting end surfaces 150 and 151 are to be formed. Further,an SiO₂ (silicon oxide) layer 132 having a thickness of 200 nm is formedall over the substrate 100.

[0088] Next, annealing (first annealing) is carried out at a temperatureof 510° C. for two hours, whereby Zn atoms are diffused from the ZnO(zinc oxide) layer 131 to the cap layer 107 and the second uppercladding layer 106 along the regions where the light-emitting endsurfaces 150 and 151 are to be formed. Under this annealing condition,the p-type dopant, Be atoms, hardly diffuse in regions other than theregions located near the light-emitting end surfaces 150 and 151.

[0089] What is important in this case is that no intermixing takes placein the etching stopper layer 104 and the active layer 102 through thisfirst annealing process. For example, FIG. 5 shows the amount of thephotoluminescence wavelength shift of the active layer (indicated by •in FIG. 5) when the temperature of the first annealing process isvaried. It was confirmed that the photoluminescence wavelength of theactive layer at the time of the end of the first annealing exhibitedonly a shift of 2 nm to the shorter wavelength side in comparison withthe photoluminescence wavelength before the first annealing. Moreover,the function of the etching stopper layer 104 to stop etching in theetching process, which will be described below, was maintained normallywhen the amount of wavelength shift of the active layer 102 through thisfirst annealing process was not greater than 15 nm. However, thefunction was lost when the amount of wavelength shift of the activelayer 102 through this first annealing process exceeded 15 nm.

[0090] The SiO₂ layer 132 and the ZnO layer 131 are removed by bufferedhydrofluoric acid, and thereafter, the cap layer 107 and the p-typeupper cladding layer 103 are selectively etched by a mixed solution ofsulfuric acid, hydrogen peroxide and water, and sulfuric acid,respectively, as shown in FIG. 4C, to thereby form a ridge 105, which isconstructed of parts of the cap layer 107 and the p-type upper claddinglayer 103, has a bottom width of 4 μm and extends in a stripe shapeperpendicularly to the light-emitting end surfaces 150 and 151. In thisetching process, the p-type etching stopper layer 104 is substantiallynot etched by sulfuric acid, and therefore, the p-type first uppercladding layer 103, which exists under the layer 104, is not etched.

[0091] As shown in FIG. 4D, an n-type Al_(0.5)In_(0.5)P current blockinglayer 108 is formed in the whole region of the substrate 100 by the MBEmethod.

[0092] Subsequently, in order to prevent Zn atoms diffused in the ridge105 from being discharged to the outside during the second annealingprocess described next, an SiO₂ (silicon oxide) layer 130, which has athickness of 500 nm, is formed as an impurity evaporation preventinglayer on the current blocking layer 108.

[0093] Subsequently, in a nitrogen ambient, annealing (second annealing)is carried out at a temperature of 680° C. for one hour. Through thisprocess, the Zn atoms diffused in the regions of the ridge 105 in whichthe light-emitting end surfaces 150 and 151 are to be formed are againdiffused now to the active layer 102 through the etching stopper layer104, whereby local intermixing of the etching stopper layer 104 and theactive layer 102 takes place at those portions 104B and 102B that extendalong the light-emitting end surfaces 150 and 151 to be formed and arelocated just under the ridge 105. As shown in FIG. 5, it was confirmedthat the photoluminescence wavelength of the intermixed portions 102B ofthe active layer 102 was shifted by 70 nm to the shorter wavelength sidewith respect to the non-intermixed inside part 102A of the active layer102. After the completion of the semiconductor laser, the intermixedportions 102B each operate as a window region of a small quantity oflaser light absorption at the light-emitting end surfaces to therebyrestrain the COD (Catastrophic Optical Damage). Under this annealingcondition, the Be atoms used as a p-type dopant scarcely diffuse intoregions other than the regions located near the light-emitting endsurfaces 150 and 151.

[0094] As shown in FIG. 4E, the SiO₂ layer 130 is removed by bufferedhydrofluoric acid. Then, a portion that exists on the ridge 105 and in aregion other than the regions near the light-emitting end surfaces 150and 151 is removed from the n-type Al_(0.5)In_(0.5)P current blockinglayer 108. That is, of all the parts of the n-type Al_(0.5)In_(0.5)Pcurrent blocking layer 108 on the substrate 100, parts 108B that existin the regions located near the light-emitting end surfaces 150 and 151as well as parts (indicated by 108) existing at both sides of the ridge105 are left intact.

[0095] Subsequently, as shown in FIG. 1, a p-type GaAs contact layer 110(having a thickness of 4 μm) is formed over the substrate 100, andelectrodes 115 and 116 are further formed on the lower and uppersurfaces, respectively, of the wafer (completion of the waferfabrication). Subsequently, the wafer is cleaved along the regions wherethe light-emitting end surfaces 150 and 151 are to be formed, i.e., in amanner that the intermixed portions 102B of the active layer 102 defineresonator end surfaces. Then, coating is performed so that the one endsurface 150 comes to have a reflectance of 8% and the end surface 151located on the opposite side comes to have a reflectance 91%, aslight-emitting end surfaces (completion of the laser chip fabrication).The resonator length was set to 800 μm, and the length of the intermixedportion 102B of the active layer 102 was set to 25 μm at both of theopposite light-emitting end surfaces 150 and 151.

[0096] Each laser chip was mounted on a stem, and the characteristicswere examined with a current applied. A maximum optical output of 265 mWwas obtained at a wavelength of 654 nm and it was confirmed that the CODwas not generated. Moreover, a leak current, which might be generated ifthe etching stopper layer was destroyed in the etching stage, was notobserved.

[0097] In order to prove this effect, a Zn concentration profile in thedirection of depth in the region where the ridge 105 exists was measuredby SIMS (secondary ion mass spectrometry). The width of the ridge 105was set to 500 μm for the sake of convenience of the SIMS analysis. FIG.8 shows the Zn concentration profiles obtained after the first annealingat a temperature of 510° C. for two hours and the second annealing. FIG.8 indicates that Zn atoms do not reach the etching stopper layer 104when the first annealing ends, and that Zn atoms diffuse beyond theactive layer 102 after the second annealing ends.

[0098]FIG. 5 and FIG. 3 show the results of the photoluminescencewavelength shift of the active layer under the conditions such asannealing conditions and so on of the present embodiment. In this case,due to the fact that the photoluminescence of the active layer is moreeasily measured than the photoluminescence of the etching stopper layerand the fact that the active layer and the etching stopper layer areintermixed to the same extent since the layers are separated apart byonly a distance of 0.2 μm that is the thickness of the first uppercladding layer, the function of the etching stopper layer is evaluatedthrough the photoluminescence of the active layer. A broken line isdrawn at a critical active layer photoluminescence wavelength shiftvalue of 15 nm. The etching stopper layer 104 has the function to stopthe etching when the wavelength shift is not greater than the value of15 nm, and the etching stopper layer 104 does not have the function tostop the etching when the wavelength shift exceeds 15 nm. As shown inFIG. 5, when the SiO₂ layer 130 is provided as an impurity evaporationpreventing layer, the wavelength shift is largely increased after thesecond annealing than after the first annealing. It can be understoodthat the wavelength shift becomes 18 nm or more after the secondannealing when the first annealing temperature is not lower than 450° C.

[0099] On the other hand, as shown in FIG. 6, when the SiO₂ layer 130 asan impurity evaporation preventing layer is not provided, the wavelengthshift slightly increases even after the second annealing than after thefirst annealing. It can be understood that a wavelength shift amount of18 nm of the active layer is obtained with the function of the etchingstopper layer maintained when the first annealing temperature is set at,for example, 520° C. According to this fact, with the annealingtemperature and time set at 520° C. and two hours as the first annealingconditions, a comparative example having no SiO₂ layer 130 as animpurity evaporation preventing layer was fabricated. In this case, thephotoluminescence wavelength of the active layer became 18 nm after thesecond annealing, and the COD took place at a maximum optical output of174 mW. However, it was confirmed that the maximum optical output wasimproved in comparison with the case where no window layer was provided(in which case maximum optical output is 120 mW). FIG. 9 shows arelation between the amount of wavelength shift of the active layer andthe maximum optical output. The semiconductor laser of the presentembodiment is intended mainly to increase the writing speed to DVD-R andDVD-RW of the next generation. Because it is required to double thewriting speed every generation, the optical output of the correspondingsemiconductor laser should be increased to 1.41 times that of theconventional device. FIG. 9 indicates that this requirement is satisfiedwhen the amount of wavelength shift is not smaller than 18 nm.

[0100] Although the SiO₂ layer 130 is adopted as the impurityevaporation preventing layer in the present embodiment, the presentinvention is not limited to this. Silicon nitride or alumina can also beadopted as the impurity evaporation preventing layer. These materialscan suitably be used as a fine or dense masking material for preventingZn from evaporating to the outside, similarly to SiO₂. Semiconductorlasers were actually fabricated by adopting each of silicon nitride andalumina for the impurity evaporation preventing layer. The amount ofwavelength shift after the second annealing was 75 nm in the case ofsilicon nitride and 73 nm in the case of alumina. Moreover, similarly toSiO₂, the silicon nitride and alumina can be removed with hydrofluoricacid or the like without etching the compound semiconductor underlayer.

[0101] Moreover, the p-type dopant was provided by Be in the presentembodiment. However, even by employing another dopant of a smalldiffusion coefficient, in particular, C, satisfactory devicecharacteristics (laser oscillation threshold value, etc.) can beobtained with the diffusion of the p-type dopant into the active layerrestrained.

[0102] Moreover, sulfuric acid was used as an etchant for the secondupper cladding layer in the present embodiment. However, even phosphoricacid or hydrochloric acid, which etch the second upper cladding layerbut hardly etch the etching stopper layer (etching rates of the twolayers are different by ten times or more), is therefore able to bepreferably used.

[0103] Moreover, although an ordinary annealing furnace was used for theannealing process in the present embodiment, an RTA (rapid thermalannealing) furnace can also be used. RTA means annealing with anextremely rapid temperature increase of 10° C./sec. to 100° C./sec. Inthis case, it is proper to set a temperature retention time at about 20seconds to 10 minutes. Since the retention time is short, it is requiredto raise the temperature higher than the normal annealing temperature.

[0104]FIG. 7 shows the results of the photoluminescence wavelength shiftof the active layer when the first annealing time is changed to 10minutes and to 20 seconds. It can be understood that the wavelengthshift becomes 15 nm or less at a first annealing temperature of nothigher than 570° C. when the first annealing time is 10 minutes. Whenthe temperature retention time is 20 seconds, it is proper to carry outthe first annealing at a temperature of 550° C. to 650°0 C. and carryout the second annealing at a temperature of 650° C. to 850° C. RTA,which has the advantage that the working hours are short, canappropriately be used if attention is paid to the temperatureuniformity.

[0105] (Second Embodiment)

[0106]FIG. 10 shows the device structure of an end-surface window typesemiconductor laser of the second embodiment. FIG. 11 shows a crosssection taken along the line XI-XI of FIG. 10, and FIG. 12 shows a crosssection taken along the line XII-XII of FIG. 10.

[0107] As shown in FIG. 10, a lower cladding layer 201, an active layer202 for generating laser light, a first upper cladding layer 203 and anetching stopper layer 204 are stacked in this order on an n-type GaAssubstrate 200. The active layer 202 is formed by alternating undopedquantum well layers and barrier layers. A ridge 205, which extends in astripe shape perpendicularly to light-emitting end surfaces 250 and 251,is constructed of a second upper cladding layer 206 and a p-type GaAscap layer 207. An n-type AlInP current blocking layer 208 and an n-typeGaAs current blocking layer 209 are formed in regions at both sides ofthe ridge 205. As is apparent from FIG. 11, these current blockinglayers 208 and 209 extend also over the ridge 205 (the extended portionsare indicated by 208B and 209B) in regions located near thelight-emitting end surfaces 250 and 251 and cover those portions 202B ofthe active layer 202 that are located near the light-emitting endsurfaces 250 and 251. With this arrangement, a wattless current isprevented from being injected into the portions 202B of the active layer202 that are located near the light-emitting end surfaces 250 and 251.Moreover, as is apparent from FIGS. 11 and 12, with regard to the XI-XIdirection, the p-type GaAs cap layer 207 and a p-type GaAs contact layer210 are in contact and electrically connected with each other in aninternal region other than the regions located near the light-emittingend surfaces 250 and 251.

[0108] In this semiconductor laser, regarding regions extending alongthe light-emitting end surfaces 250 and 251, the portions 202B of theactive layer 202 that are located just under the ridge 205 have anenergy bandgap greater than the energy bandgap of the other portions ofthe active layer 202 that are located in positions corresponding to bothsides of the ridge 205. Thus, the portions 202B serve as window regionsabsorbing little laser light. The portions 202B are formed by diffusingthe diffused impurity, which has been contained in the second uppercladding layer 206 of the ridge 205, as described later. Therefore, inthe active layer 202, the bandgap becomes gradually reduced away fromthe ridge 205. The light density of the laser light is also graduallyreduced away from the ridge 205, and this is advantageous from theviewpoint of restriction of photoabsorption.

[0109] This semiconductor laser is fabricated as follows.

[0110] First of all, as shown in FIG. 13A, on the n-type GaAs substrate200, there are formed by the MOCVD method (metal-organic chemical vapordeposition method) an n-type (Al_(0.7)Ga_(0.3))_(0.5)In_(0.5)P lowercladding layer 201 (having a carrier density of 1×10¹⁸ cm⁻³), an activelayer 202 that has a structure in which three undoped GaInP layers(having a thickness of 6 nm) alternate with four undoped(Al_(0.5)Ga_(0.5))_(0.5)In_(0.5)P layers (having a thickness of 8 nm)with one undoped GaInP layer being held between adjacent(Al_(0.5)Ga_(0.5))_(0.5)In_(0.5)P layers, a p-type(Al_(0.7)Ga_(0.3))_(0.5)In_(0.5)P first upper cladding layer 203 (havinga carrier density of 0.7×10¹⁸ cm⁻³), a p-type Ga_(0.6)In_(0.4)P etchingstopper layer 204 (having a carrier density of 1.5×10¹⁸ cm⁻³), a p-type(Al_(0.7)Ga_(0.3))_(0.5)In_(0.5)P second upper cladding layer 206(having a carrier density of 2×10¹⁸ cm⁻³) and a p-type GaAs cap layer207 (having a carrier density of 3×10¹⁸ cm⁻³) in this order. In thiscase, the n-type dopant is Si, and the p-type dopant is C for the firstupper cladding layer and Zn for the other p-type layers.

[0111] Next, as shown in FIG. 13B, a ZnO (zinc oxide) layer 231 having athickness of 50 nm, which serves as an impurity diffusion source, isformed in a stripe shape on the cap layer 207 along the regions wherethe light-emitting end surfaces 250 and 251 are to be formed. Further,an SiO₂ (silicon oxide) layer 232 having a thickness of 200 nm is formedin the whole region on the substrate 200.

[0112] Next, annealing (first annealing) is carried out at a temperatureof 510° C. for two hours, whereby Zn atoms are diffused from the ZnO(zinc oxide) layer 231 to the cap layer 207 and the second uppercladding layer 206 along the regions where the light-emitting endsurfaces 250 and 251 are to be formed. What is important in this case isthat neither the GaInP etching stopper layer 204 nor the active layer202 is intermixed with Zn atoms through this first annealing process.

[0113] The SiO₂ layer 232 and the ZnO layer 231 are removed by bufferedhydrofluoric acid, and thereafter, the cap layer 207 and the p-typeupper cladding layer 203 are selectively etched by a mixed solution ofsulfuric acid, hydrogen peroxide and water, and sulfuric acid,respectively, as shown in FIG. 13C, to thereby form a ridge 205, whichis constructed of parts of the cap layer 207 and the p-type uppercladding layer 203, has a bottom width of 3 μm and extends in a stripeshape perpendicularly to the light-emitting end surfaces 250 and 251. Inthis etching process, the p-type etching stopper layer 204 issubstantially not etched by sulfuric acid, and therefore, the p-typefirst upper cladding layer 203, which exists under the layer 204, is notetched, either.

[0114] As shown in FIG. 13D, an n-type Al_(0.5)In_(0.5)P currentblocking layer 208 (0.6 μm) and an n-type GaAs current blocking layer209 (2.0 μm) are formed in this order over the substrate 200 by the MBEmethod.

[0115] In this case, the n-type Al_(0.5)In_(0.5)P current blocking layer208 is grown at a substrate temperature of 490° C. and also functions asan impurity evaporation preventing layer for preventing the impuritydiffusion to the outside. Moreover, in growing the n-type GaAs currentblocking layer 209, the substrate temperature is set at 490° C. at thestart of growth of the n-type GaAs layer, and is raised up to 630° C.after the n-type GaAs layer has been grown by about 0.2 μm. By thusraising the substrate temperature to 630° C., the Zn atoms diffused inthe regions of the ridge 205 in which the light-emitting end surfaces250 and 251 are to be formed are again diffused now to the active layer202 through the etching stopper layer 204, whereby local intermixing ofthe etching stopper layer 204 and the active layer 202 takes place atthose portions 204B and 202B that extend along the light-emitting endsurfaces 250 and 251 to be formed and are located just under the ridge205. It was confirmed that the photoluminescence wavelength of theintermixed portions 202B of the active layer 202 was actually shifted by45 nm to the shorter wavelength side with respect to the inside 202Awhere no intermixing took place. After the completion of thesemiconductor laser, the intermixed portions 202B each operate as awindow region of a small quantity of laser light absorption at thelight-emitting end surfaces to thereby restrain the COD.

[0116] As shown in FIG. 15B, portions that exist on the ridge 205 and inregions other than the regions near the light-emitting end surfaces 250and 251 are removed from the n-type GaAs current blocking layer 209 andthe n-type Al_(0.5)In_(0.5)P current blocking layer 208. That is, of allportions of the n-type GaAs current blocking layer 209 and the n-typeAl_(0.5)In_(0.5)P current blocking layer 208 on the substrate 100,portions 209B, 208B that exist in the regions located near thelight-emitting end surfaces 250 and 251 as well as portions (indicatedby 209, 208) existing at both sides of the ridge 205 are left intact.

[0117] Subsequently, as shown in FIG. 11, a p-type GaAs contact layer210 (having a thickness of 4 μm) is formed over the substrate 200, andelectrodes 215 and 216 are further formed on the lower and uppersurfaces, respectively, of the wafer (completion of the waferfabrication). Subsequently, the wafer is cleaved along the regions wherethe light-emitting end surfaces 250 and 251 are to be formed, i.e., in amanner that the portions 202B of the active layer 202 in whichintermixing took place define resonator end surfaces. Then, coating isperformed so that the one end surface 250 comes to have a reflectance of8% and the end surface 251 located on the opposite side comes to have areflectance of 91%, as light-emitting end surfaces (completion of thelaser chip fabrication). The length of the intermixed portion 202B ofthe active layer 202 is 20 μm at both of the light-emitting end surface250 and the end surface 251 located on the opposite side.

[0118] Each laser chip was mounted on a stem, and the characteristicswere examined with a current applied. A maximum optical output of 225 mWwas obtained at a wavelength of 656 nm and it was confirmed that the CODwas not generated. Moreover, a leak current, which might be generated ifthe etching stopper layer was destroyed in the etching stage, was notobserved.

[0119] In the present embodiment, the n-type Al_(0.5)In_(0.5)P currentblocking layer 208 was first formed at a comparatively low temperature,and then the n-type GaAs current blocking layer 209 was formed at thesubstrate temperature for the second annealing. However, the substratetemperature for the second annealing may be provided at some pointduring the formation of the n-type Al_(0.5)In_(0.5)P current blockinglayer 208. By thus performing the second annealing utilizing thesubstrate temperature in the current blocking layer forming stage, thefabricating process can be simplified than in the first embodiment. As acurrent blocking layer forming method, it is acceptable to use the MOCVDmethod besides the MBE method.

[0120] (Third Embodiment)

[0121]FIG. 14 shows the device structure of an end-surface window typesemiconductor laser according to a third embodiment of the presentinvention. FIG. 15 shows a cross section taken along the line XV-XV ofFIG. 14, and FIG. 16 shows a cross section taken along the line XVI-XVIof FIG. 14.

[0122] As shown in FIG. 14, an n-type AlGaAs lower cladding layer 301,an active layer 302 for generating laser light, a p-type AlGaAs firstupper cladding layer 303, a p-type GaAs etching stopper layer 304(having a thickness of 3 nm) are stacked in this order on an n-type GaAssubstrate 300. The active layer 302 is formed by alternating undopedquantum well layers and barrier layers. A ridge 305, which extends in astripe shape perpendicularly to light-emitting end surfaces 350 and 351,is constructed of a p-type AlGaAs second upper cladding layer 306 and ap-type GaAs cap layer 307. An n-type AlGaAs current blocking layer 308is formed in regions on both sides of the ridge 305. As is apparent fromFIG. 15, this current blocking layer 308 exists together with a p-typeGaAs layer 309 also on the ridge 305 in the regions located near thelight-emitting end surfaces 350 and 351 (these portions are indicated by308B and 308B) and covers those portions 302B of the active layer 302that are located near the light-emitting end surfaces 350 and 351. Withthis arrangement, a wattless current is prevented from being injectedinto the portions 302B of the active layer 302 that are located near thelight-emitting end surfaces 350 and 351. Moreover, as is apparent fromFIGS. 15 and 16, the p-type GaAs cap layer 307 and the p-type GaAscontact layer 310 are in contact and electrically connected with eachother in an internal region other than the regions located near thelight-emitting end surfaces 350 and 351 with regard to the XV-XVdirection.

[0123] In this semiconductor laser, regarding regions of the activelayer 302 extending along the light-emitting end surfaces 350 and 351,the portions 302B of the active layer 302 that are located just underthe ridge 305 have an energy bandgap greater than the energy bandgap ofthe other portions of the active layer 302 that are located in positionscorresponding to both sides of the ridge 305. Thus, the portions 302Bserve as window regions absorbing little laser light.

[0124] This semiconductor laser is fabricated as follows.

[0125] First of all, as shown in FIG. 17A, on an n-type GaAs substrate300, there are formed by the MBE method an n-type Al_(0.5)Ga_(0.5)Aslower cladding layer 301 (having a carrier density of 1×10¹⁸ cm⁻³), anactive layer 302 that has a structure in which two undoped GaAs layers(having a thickness of 10 nm) alternate with three undopedAl_(0.3)Ga_(0.7)As layers (having a thickness of 8 nm) with one undopedGaAs layer being held between adjacent two undoped Al_(0.3)Ga_(0.7)Aslayers, a p-type Al_(0.5)Ga_(0.5)As first upper cladding layer 303(having a carrier density of 1.0×10¹⁸ cm⁻³), a p-type GaAs etchingstopper layer 304 (having a carrier density of 2.0×10¹⁸ cm⁻³), a p-typeAl_(0.5)Ga_(0.5)As second upper cladding layer 306 (having a carrierdensity of 2.5×10¹⁸ cm⁻³) and a p-type GaAs cap layer 307 (having acarrier density of 3×10¹⁸ cm⁻³) in this order. In this case, the n-typedopant is Si, and the p-type dopant is Be.

[0126] Next, as shown in FIG. 17B, a ZnO (zinc oxide) layer 331 having athickness of 50 nm, which serves as an impurity diffusion source, isformed in a stripe shape along the regions where the light-emitting endsurfaces 350 and 351 are to be formed. Further, an SiO₂ (silicon oxide)layer 332 having a thickness of 200 nm is formed in the whole region onthe substrate 300.

[0127] Next, annealing (first annealing) is carried out at a temperatureof 680° C. for two hours, so that Zn atoms are diffused from the ZnO(zinc oxide) layer 331 to the cap layer 307 and the second uppercladding layer 306 along the regions where the light-emitting endsurfaces 350 and 351 are to be formed. Under this annealing condition,Be atoms, used as a p-type dopant, hardly diffuse in regions other thanthe regions located near the light-emitting end surfaces 350 and 351.

[0128] The SiO₂ layer 332 and the ZnO layer 331 are removed by bufferedhydrofluoric acid, and thereafter, the cap layer 307 and the p-typeupper cladding layer 303 are selectively etched by a mixed solution ofsulfuric acid, hydrogen peroxide solution and water, and sulfuric acid,respectively, as shown in FIG. 17C, to thereby form a ridge 305 fromparts of the cap layer 307 and the p-type upper cladding layer 303, theridge 305 having a bottom width of 4 μm and extending in a stripe shapeperpendicularly to the light-emitting end surfaces 350 and 351. In thisetching process, the p-type etching stopper layer 304 is substantiallynot etched by sulfuric acid, and therefore, the p-type first uppercladding layer 303, which exists under the layer 304, is not etched,either.

[0129] As shown in FIG. 17D, an n-type Al_(0.7)Ga_(0.3)As currentblocking layer 308 and a p-type GaAs layer 309 are formed on both sidesof the ridge 305 by the MOCVD method. These layers also cover the upperportion of the ridge 305.

[0130] Subsequently, in order to prevent the Zn atoms diffused in theridge 305 from being discharged to the outside during the secondannealing process described next, an SiO₂ (silicon oxide) layer 330,which has a thickness of 500 nm, is formed as an impurity evaporationpreventing layer on the p-type GaAs layer 309.

[0131] Subsequently, in an nitrogen ambient, RTA (second annealing) iscarried out at a temperature of 950° C. for one minute. Through thisprocess, the Zn atoms diffused in those regions of the ridge 305 thatare to form the light-emitting end surfaces 350 and 351, are diffusedagain to the active layer 302 through the etching stopper layer 304,whereby local intermixing of the etching stopper layer 304 and theactive layer 302 takes place at those portions 304B and 302B that extendalong the light-emitting end surfaces 350 and 351 to be formed and arelocated just under the ridge 305. After the completion of thesemiconductor laser, the intermixed portions 302B each operate as awindow region at the light-emitting end surfaces absorbing little laserlight to thereby restrain the COD. Under this annealing condition, Beatoms used as a p-type dopant scarcely diffuse into regions other thanthe regions located near the light-emitting end surfaces 350 and 351.Accordingly, there is little possibility of raising the laseroscillation threshold due to the Be atoms reaching the active layer 302.

[0132] As shown in FIG. 17E, the SiO₂ layer 330 is removed by bufferedhydrofluoric acid. Then, portions existing on the ridge 305 and inregions other than the regions located near the light-emitting endsurfaces 350 and 351 are removed from the n-type Al_(0.7)Ga_(0.3)Ascurrent blocking layer 308 and the p-type GaAs layer 309. That is, ofall potions of the n-type Al_(0.7)Ga_(0.3)As current blocking layer 308and the p-type GaAs layer 309, the portions 309B and 308B that exist inthe regions located near the light-emitting end surfaces 350 and 351 aswell as the portions (indicated by 309 and 308) that exist on both sidesof the ridge 305 are left intact.

[0133] Subsequently, as shown in FIG. 14, a p-type GaAs contact layer310 (having a thickness of 4 μm) is formed over the substrate 300, i.e.,the wafer, and electrodes 315 and 316 are further formed on the lowerand upper surfaces, respectively, of the wafer (completion of the waferfabrication). Subsequently, the wafer is cleaved along the regions wherethe light-emitting end surfaces 350 and 351 are to be formed, i.e., in amanner that the portions 302B of the active layer 302, where theintermixing took place, become resonator end surfaces. Then, coating isperformed so that the one end surface 350 comes to have a reflectance of12% and the end surface 351 located on the opposite side comes to have areflectance of 95%, as light-emitting end surfaces (completion of thelaser chip fabrication). The resonator length was set to 800 μm, and thelength of the intermixed portions 302B of the active layer 302 was setto 25 μm at both of the light-emitting end surface 350 and the endsurface 351 located on the opposite side.

[0134] Each laser chip was mounted on a stem, and the characteristicswere examined with a current applied. A maximum optical output of 325 mWwas obtained at a wavelength of 786 nm and it was confirmed that COD wasnot generated. Moreover, a leak current, which might be generated whenthe etching stopper layer was destroyed in the etching stage, was notobserved.

[0135] The p-type dopant was Be in the present embodiment. However, evenby employing another dopant of a small diffusion coefficient, inparticular, C, satisfactory device characteristics (laser oscillationthreshold value, etc.) can be obtained with the diffusion of the p-typedopant into the active layer restrained.

[0136] Moreover, the p-type etching stopper layer 304 was made of GaAsin the present embodiment. However, when hydrofluoric acid or bufferedhydrofluoric acid is used as an etchant, no erosion will occur even withAl_(x)Ga_(1−x)As of a crystal mixture ratio x of not higher than 0.3.When Al_(x)Ga_(1−x)As (x≦0.3) is adopted as a material for the etchingstopper layer, the bandgap of the etching stopper layer increases as thevalue of x increases, so that the rate of reabsorbing light, generatedin the active layer, is reduced.

[0137] Moreover, an ordinary annealing furnace was used for the firstannealing and RTA was used for the second annealing in the presentembodiment. However, this combination is arbitrary or optional.

[0138] Moreover, although the active layer 302 was of AlGaAs in thepresent embodiment, the present invention is not limited to this. Bysandwiching an InGaAs quantum well layer by barrier layers of GaAs orAlGaAs, the oscillation wavelength may be set at, for example, 980 nm.

[0139] Even when materials other than the materials described inconnection with each of the aforementioned embodiments, the presentinvention can generally be applied to the window type semiconductorlasers that have an etching stopper layer. It is also possible tocombine compositions that do not contain Al, which are considered tohardly cause COD, such as a combination of GaInAsP cladding layers withan InGaAs active layer.

[0140] The invention being thus described, it will be obvious that thesame may be varied in many ways. Such variations are not to be regardedas a departure from the spirit and scope of the invention, and all suchmodifications as would be obvious to one skilled in the art are intendedto be included within the scope of the following claims.

What is claimed is:
 1. A semiconductor laser, which emits laser lightthrough a light-emitting end surface, comprising: a lower claddinglayer, an active layer for generating laser light, a first uppercladding layer and an etching stopper layer stacked in this order on asubstrate; a second upper cladding layer formed in a shape of a ridge onthe etching stopper layer, the ridge extending perpendicularly to thelight-emitting end surface; a current blocking layer disposed in regionson both sides of the second upper cladding layer; and an impuritydiffused in a portion extending along the light-emitting end surfacefrom the etching stopper layer to the active layer and located at leastunder the ridge for local intermixing in this portion to restrain laserlight absorption, wherein in a region along the light-emitting endsurface, the etching stopper layer has a bandgap smaller in portionsthereof disposed in positions corresponding to both sides of the ridgethan in a portion thereof located just under the ridge.
 2. Thesemiconductor laser as claimed in claim 1, wherein in the region alongthe light-emitting end surface, the active layer has a bandgap larger ina portion thereof located just under the ridge than in portions thereofdisposed in positions corresponding to both sides of the ridge.
 3. Thesemiconductor laser as claimed in claim 2, wherein in the region alongthe light-emitting end surface, a photoluminescence wavelength shift toa shorter wavelength side due to the local intermixing of the activelayer in the portion located just under the ridge is 18 nm or more, anda photoluminescence wavelength shift to the shorter wavelength side dueto the local intermixing of the active layer in the portionscorresponding to both sides of the ridge is not larger than 15 nm. 4.The semiconductor laser as claimed in claim 1, wherein the first uppercladding layer contains a diffused impurity of Be or C, and the impuritydiffused in said portion extending along the light-emitting end surfacefrom the etching stopper layer to the active layer is Zn.
 5. Thesemiconductor laser as claimed in claim 4, wherein the second uppercladding layer contains a diffused impurity of Be or C.
 6. Thesemiconductor laser as claimed in claim 1, wherein the active layercomprises at least one quantum well layer and barrier layers alternatingwith the quantum well layer; the at least one quantum well layer isconstructed of (Al_(x)Ga_(1−x))_(y)In_(1−y)P (0≦x≦1 and 0≦y≦1); and thebarrier layers are constructed of (Al_(x)Ga_(1−x))_(y)In_(1−y)P (0≦x≦1and 0≦y≦1) whose Al content (x) is greater than that of the quantum welllayer.
 7. The semiconductor laser as claimed in claim 6, wherein theetching stopper layer is constructed of Ga_(y)In_(1−y)P (0≦y≦1); and thefirst and second upper cladding layers are each constructed of(Al_(x)Ga_(1−x))_(y)In_(1−y)P (0≦x≦1 and 0≦y≦1).
 8. The semiconductorlaser as claimed in claim 1, wherein the active layer comprises at leastone quantum well layer and barrier layers alternating with the quantumwell layer; the at least one quantum well layer is constructed ofIn_(z)Ga_(1−z)As (0≦z≦1) or Al_(x)Ga_(1−x)As (0≦x≦1); and the barrierlayers are constructed of Al_(x)Ga_(1−x)As (0≦x≦1) whose Al content (x)is greater than that of the quantum well layer when the latter isconstructed of Al_(x)Ga_(1−x)As (0≦x≦1).
 9. The semiconductor laser asclaimed in claim 8, wherein the etching stopper layer is constructed ofAl_(x)Ga_(1−x)As (0≦x≦0.3), and the first and second upper claddinglayers are each constructed of Al_(y)Ga_(1−y)As (x<y≦1).
 10. A methodfor fabricating a semiconductor laser that emits laser light through alight-emitting end surface, comprising: a process for forming at least alower cladding layer, an active layer for generating laser light, afirst upper cladding layer, an etching stopper layer and a second uppercladding layer in this order on a substrate; a first annealing processfor diffusing an impurity for restraining laser light absorption intothe second upper cladding layer along a region where a light-emittingend surface is to be formed, under a condition that allows the etchingstopper layer to maintain a function of stopping etching for the secondupper cladding layer; an etching process for performing etching untilthe etching stopper layer is reached such that the second upper claddinglayer is left in a ridge shape extending perpendicularly to thelight-emitting end surface to be formed; and a second annealing processfor re-diffusing the impurity once diffused in the region of theridge-shaped second upper cladding layer where the light-emitting endsurface is to be formed, into the active layer through the etchingstopper layer to thereby cause local intermixing of the active layer ina portion that extends along the light-emitting end surface to be formedand is located just under the ridge.
 11. The semiconductor laserfabricating method as claimed in claim 10, wherein a photoluminescencewavelength shift to a shorter wavelength side through the firstannealing process at a portion of the active layer that extends alongthe region where the light-emitting end surface is to be formed is notlarger than 15 nm, and a photoluminescence wavelength shift to theshorter wavelength side through the second annealing process at theportion of the active layer that extends along the region where thelight-emitting end surface is to be formed and is located just under theridge is 18 nm or larger.
 12. The semiconductor laser fabricating methodas claimed in claim 10, further comprising: after the etching processand before the second annealing process, providing on the substrate animpurity evaporation preventing layer for preventing the impurity fromevaporating from the second upper cladding layer to the outside.
 13. Thesemiconductor laser fabricating method as claimed in claim 12, whereinthe impurity evaporation preventing layer is made of silicon oxide,silicon nitride, or alumina.
 14. The semiconductor laser fabricatingmethod as claimed in claim 12, wherein the impurity evaporationpreventing layer is made of a compound semiconductor layer.
 15. Thesemiconductor laser fabricating method as claimed in claim 14, whereinthe impurity evaporation preventing layer is formed at a temperaturelower than a temperature at which the impurity is re-diffused in thesecond annealing process.
 16. The semiconductor laser fabricating methodas claimed in claim 10, wherein the first upper cladding layer containsa diffused impurity of Be or C; and the impurity diffused in the portionof the active layer that extends along the region where thelight-emitting end surface is to be formed and is located just under theridge is Zn.
 17. The semiconductor laser fabricating method as claimedin claim 16, wherein the diffused impurity contained in the second uppercladding layer is Be or C.
 18. The semiconductor laser fabricatingmethod as claimed in claim 10, wherein the active layer is formed byalternating at least one quantum well layer and barrier layers; the atleast one quantum well layer is constructed of(Al_(x)Ga_(1−x))_(y)In_(1−y)P (0≦x≦1 and 0≦y≦1); the barrier layers areconstructed of (Al_(x)Ga_(1−x))_(y)In_(1−y)P (0≦x≦1 and 0≦y≦1) whose Alcontent (x) is greater than that of the quantum well layer; the firstannealing process is carried out under a condition of a temperature of450° C. to 570° C. for 10 minutes or more or a temperature of 550° C. to650° C. for 10 minutes or less; and the second annealing process iscarried out under a condition of a temperature of 570° C. to 750° C. for10 minutes or more or a temperature of 650° C. to 850° C. for 10 minutesor less.
 19. The semiconductor laser fabricating method as claimed inclaim 10, wherein the second annealing process is carried out by raisinga substrate temperature when a semiconductor layer is formed bymolecular beam epitaxy.
 20. The semiconductor laser fabricating methodas claimed in claim 10, wherein the second annealing process is carriedout in a nitrogen ambient.